Not at all. its the total length of the bullet that makes it require a specific RPM, and Bullet RPM is a fuction of velocity and rifling pitch.

Take the 165 Hdy IB I listed above VS the 180 gr Failsafe also listed above. The failsafe requires 13.2 twist (180 gr bullet measuring 1.303" at 2700 fps, which is .30-06 velocity for that weight) and the IB requires 13.4 twist (165 gr bullet measuring 1.312 at 2800 fps, which is also .30-06 velocity for that weight) and the Failsafe has almost twice the bearing surface because it doesnt have a Boat tail. Now slow them both down in reduced loads to 2500 fps and the FS requires 12.7 and the IB requires 12.7, so internal ballistcally speaking they are equals in thier requirements as far as barrel twist is concerned.

how do we determine the specific gravity of said projectile? Sierra's website does not list that data

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Specific gravity is the ratio of the density of the subject object to the density of water. Since the density of water is, by definition, 1.00, the density of the bullet in question is the specific gravity.

To measure it, fill a graduated cylinder with water to some level less than full, and record the level. This will be in mL, but at room temperature a mL is the same as a cc. Drop in a bullet and record the new level. Since all but really weird bullets sink, this new level, minus the previous level, is the volume of the bullet.

Divide the weight of the bullet by its volume, and you have its density. Of course, bullet weights are given in grains, and densities are reported in grams per cubic centimeter, so we have to do a bit of math, but it's not hard. Since a pound is 7000 grains, the weight of the bullet divided by 7000 is its weight in pounds. And since one pound is about 454 grams, multiplying the bullet weight in pounds by 454 yields its weight in grams. A simple division of the weight in grams by the volume in cubic centimeters (ccs) yields the density of a bullet.

Of course, most normal people don't own a graduated cylinder - I do, but I'm a bit odd - so this isn't terribly practical. Another option is to fill a standard measuring cup with water about halfway and record the level. Then toss in a hundred bullets and record the change in level. The difference will be the volume of 100 bullets, so you can simply divide by 100 to get the volume of one bullet.

I have to ask, though, why anyone would care about the specific gravity of a bullet. It's essentially a density problem, and there is very little variation in density among all the various bullet types of a given weight. A 100 gr hard lead bullet will have a density only slightly higher than a copper-clad FMJ bullet of the same weight, and I really doubt that the difference will have any influence on anything important.